Bump up version of aws-sdk dependency
Accept and pass :credential_provider option to AWS-SDK
Make configuration of :s3_credentials optional

Since AWS-SDK 1.5.3 [1] the library automatically checks for credentials.
This makes it unnecessary to supply credentials when they are present
in environment variables or supplied via an IAM Instance Profile.

[1] http://aws.amazon.com/releasenotes/Ruby/0304005114813505
Add test for supplying :s3_credentials in an unsupported manor
Lower requirement of aws-sdk to 1.5.7